Not Applicable.
1. Field of the Invention
The present invention relates to the formation of stable multi-layer nonlinear optical polymer (NLOP) films. More particularly, the invention relates to a process of second-order nonlinear optical polymer films which are formed by a solution deposition scheme which results in an electro-optic (EO) film. Still more particularly, the electro-optic films do not require electric-field poling nor undergo high temperature processing treatment.
2. Description of the Related Art
The art of organic polymeric thin films for photonic applications has been a rapidly evolving area of research. One class of materials within this field, NLOP films, has potential for breakthroughs in low cost integrated devices for the telecommunication and data-communication industries. NLOP films are desired in the fabrication of electro-optic (EO) waveguides. This application of the nonlinear optical (NLO) films permits optical signals guided in the films to be switched from one path to another and the phase or amplitude of an optical signal to be modulated at greater than 40 GHz. NLOP films also are desired for sum-difference frequency generation, such as frequency-doubling. Chromophore alignment is stable, remaining constant for several years.
Nonlinear optical polymers are molecular structures which contain chemically attached asymmetric chromophores, also called dyes. Optical nonlinearity is caused by the electrical polarization and polarizability of the chromophore interacting with the electric field of the electromagnetic radiation. Second-order nonlinearity normally occurs in non-centrosymmetric chromophoric films. Asymmetric chromophores must be at least partially aligned in the same direction, called polar alignment, to cause second-order nonlinearity in the NLOP film. See U.S. Pat. No. 5,247,055 issued Sep 21, 1993 to Stenger-Smith et al., U.S. Pat. No. 5,520,968, issued May 28, 1996 to Wynne et al., and the book, Polymers for Second-Order Nonlinear Optics, G. A. Lindsay and K. D. Singer, Eds., Am. Chem. Soc. Advances in Chemistry Series 601, Washington, D.C., 1995.
The electrical polarization in a film is the dipole moment per unit volume. The molecular structure of the chromophore and its orientation govern the nonlinear optical properties of the system. Furthermore, polymer structure facilitates the processability and enhances temporal stability of the final product.
Macroscopic optical properties of NLO films depend on the electrical polarization in the film. In order for films to have a large NLO coefficient, a high concentration of asymmetrical, highly polarizable chromophores arranged in a polar configuration must be present. Films made from nonlinear optical polymers then possess a molecular structure with polar aligned chromophores. This makes the film asymmetrically polarizable.
Polymers may be NLO-active or NLO-inactive. NLO-active polymers are defined as those polymers which have polarizable chromophores with permanent dipole moments. Polymer films that exhibit second-order NLO properties must contain NLO-active polymers with non-centrosymmetric alignment of the chromophores. NLO-inactive polymers are defined as those polymers which contain no polarizable chromophores or chromophores whose ground-state dipole moment is nil.
In the past years, several types of polymers have been developed which are effective in EO modulation of optical signals. Films made from nonlinear optical polymers generally are glassy polymers. Amorphous glassy polymers are transparent and scatter very little light.
Sidechain Polymers:
Sidechain polymers have asymmetric chromophores chemically attached at one point pendant to the backbone of the polymer. For example, the attachment occurs at the electron accepting end or at the electron donating end of the chromophore.
Mainchain Polymers:
In mainchain polymers the chromophores are chemically attached (linked) at both ends resulting in the majority of the chromophore forming part of the backbone. The unique characteristic of this class of polymers is that the asymmetric chromophores can be linked in a head-to-tail pattern (isoregic), head-to-head pattern (syndioregic), or in a random head-to-head and head-to-tail (aregic) pattern. Because chromophores in mainchain polymers are linked at both ends, the chromophores have one less degree of freedom of motion relative to sidechain polymers.
Stability:
There are a number of different types of stability relevant to asymmetrical chromophores. Physical stability refers to the stability of the polar chromophore alignment to relaxation into a nonpolar state. Chemical stability refers to the integrity of the chemical structure of the chromophore, for example, against oxidation or hydrolysis. Photochemical stability refers to the stability of the chromophore to irradiation by light, especially in the presence of oxygen and water. Temporal stability refers to how well the physical, photochemical and chemical stability are maintained at a given temperature. Finally, processing stability refers to how well the polymer handles film processing procedures and various packaging operations. All of the above types of stability are critical if long term temporal stability is to be achieved.
Several methods are known to produce the NLOP films. Two primary techniques used to impart polar order in the film are elevated temperature electric-field poling and room temperature Langmuir-Blodgett processing.
1. Electric-Field Poling:
One method of producing nonlinear optical polymers is the electric field poling of spun-cast films. Thin polymer films are prepared for poling by spin-coating a liquid solution of the polymer (about a 10 to 30% concentration) onto a solid substrate. The solvent is removed by baking the film just above the glass transition temperature (Tg). An electric field is applied across the film in one of two ways:
1) By corona poling the film on a grounded conductor plane near the film""s Tg for 1 to 150 minutes.
2) By charging two electrodes contacting the film heated to Tg for 1 to 150 minutes.
Either of these processes can create an electric field of fifty to several hundred volts/micron across the film. The film is then cooled with the field on. After the external field is removed, a net alignment of dipole moments can remain locked in the film for long periods of time, providing that the temperature of the film remains well below any solid state transition, such as the Tg. Electric-field poling removes centrosymmetry, thereby imparting polar order in a film. Generally, the nonlinear optical coefficients increase linearly with poling field until a saturation point is reached, or dielectric breakdown of the film shorts out the electrodes.
There are several problems associated with electric-field poling. First, the polymer utilized must be heated to high temperatures. At these high temperatures thermal disordering of the chromophores works against the torque of the electric field resulting in the chromophores being less well ordered. In addition, polymers containing formal mobile charges are very difficult to pole with an electric field because the charges tend to migrate through the polymer causing dielectric breakdown (i.e. shorting out the electrode).
2. Langmuir-Blodgett (LB) Processing:
Another approach in the preparation of NLOP films is the Langmuir-Blodgett (LB) deposition technique. In conventional LB processing, the polymer molecules are designed to have hydrophilic and hydrophobic groups which cause the polymer to float on the gas-liquid interface in a preferred conformation. These hydrophilic/hydrophobic forces are useful in removing the centrosymmetry by orienting the chromophores normal to the plane of the film.
To make films by LB processing, an organic compound is floated on a liquid, e.g. water, ethylene glycol or other aqueous solutions, in a trough. A solid substrate is dipped through the gas-liquid interface depositing a single molecular layer on the substrate. Thicker films comprised of multiple layers of polymers are built up by repeatedly dipping the substrate into and/or out of the trough, depositing one layer per stroke.
One of the main advantages that conventional LB processing has over electric-field poling is that LB processing may be carried out at room temperature or lower. Furthermore, unlike electric-field poling, formal ionic charges on the polymer need
Previous materials utilizing the LB methodology for the fabrication of waveguides (U.S. Pat. No. 5,162,453 issued Nov. 10, 1992 to Hall et al., U.S. Pat. No. 5,225,285 issued Jul. 6, 1993 to Hall et al., U.S. Pat. No. 4,830,952 issued May 16, 1989 to Penner et al, and U.S. Pat. No. 4,792,208 issued Dec. 20, 1988 to Ulman et al.) have suffered from thermal instability due to the presence of low melting alkyl and fluoroalkyl hydrophobic chains. One strategy to increase the thermal stability of LB films is the use of interlayer and/or intralayer covalent bonding (i.e. crosslinking). Another strategy is to attach chromophores to rigid polymer backbones. However, attaching sidechain chromophores to polyimides failed to provide stable multilayer NLOP films. See Thin Solid Films, 244 (1994) 754-757, and Langmuir, 10 (1994) 1160-1163.
A limitation of LB technology is the amount of time required to build up films of sufficient thickness ( greater than 0.5 micrometers) for waveguiding. Two ways that the rate of deposition can be increased on the substrate without sacrificing film quality are:
1) Lowering monolayer viscosity by use of higher subphase temperatures, choice of subphase ions, or change of pH. See xe2x80x9cInsoluble Monolayers at Liquid-Gas Interfacesxe2x80x9d G. L. Gaines, Interscience Publishers, New York, 1966.
2) Utilizing alternative monolayer compression schemes such as the flowing subphase. See Advanced Materials 1991, 3(1), 25-31.
As mentioned earlier, the classical LB processing technique requires that the materials self-assemble into non-centrosymmetric order at an interface between gas and aqueous liquid through a balance of hydrophobicity and hydrophilicity. Typically, by design, functional groups are introduced into the polymer chemical structure to bring about preferential chromophore orientation. These functional groups, especially the alkyl groups which are used for hydrophobicity, lead to a lowering of the Tg and a dilution of the concentration of chromophores. Dilution causes a lowering of the nonlinear optical coefficient of the waveguide.
The criteria for selecting polymers are quite different for Langmuir-Blodgett deposition and electric-field poling process. For example, mobile ionic charges should be absent for best results in electric-field poling because the ions tend to migrate in large electric fields leading to dielectric breakdown of the organic film. For Langmuir-Blodgett deposition, care must be exercised to have the proper balance of hydrophilicity and hydrophobicity. Alternating Polyelectrolyte Deposition (APD):
Alternating Polyelectrolyte Deposition (APD) is performed by alternately dipping a solid substrate in separate aqueous solutions of a polycation and a polyanion. See Thin Solid Films 1992, 210/211, 831. During the APD process, the polyelectrolyte chains in solutions are attracted electrostatically to the substrate. At full substrate coverage, the outer surface carries a net charge of the same sign as the most recently deposited polyelectrolyte. Hence, alternate dipping into solutions of polycation and polyanion, leads to a build up of a uniform film of any desired thickness. In the APD technique, a layer is defined as the result of one polyelectrolyte deposition step.
Unlike the LB technique, a layer in the APD technique can range in thickness depending on whether the polymer chain is tightly coiled or expanded. Individual layer thickness will increase with increasing ionic strength of the polyelectrolyte solution, which can be increased by adding a simple salt such as NaCl or increasing the polyelectrolyte concentration. Ions from the added salt screen the intra-chain charges thus allowing the polyelectrolyte chain to adopt a more coiled conformation. Tightly coiled chains yield a thicker deposited layer compared to polymer chains deposited with the more extended conformation characteristic of low ionic strength solutions. See Macromolecules 1993, 26, 7058-7063.
It is desired to have improvements in the field of NLO-films. The process and product of the present invention address this need. The present invention eliminates the need for electric-field poling, eliminates the dilution effect of the hydrophobic alkyl groups, eliminates the need for high temperature treatment, creates stronger ionic bonds between the polymer chains, increases the number of active bi-layers, and increases the concentration of chromophores within the bi-layers by permitting NLO-active polycation and NLO-active polyanion bi-layers.
In view of the foregoing, it is the object of this invention to alternately deposit NLO-active polycation polymer layers and NLO-active polyanion polymer layers by APD that are non-centrosymmetric and have polar order.
Additionally, it is an object of the present invention to provide a process for producing NLOP film using a solution deposition scheme which results in an electro-optic (EO) film which is not required to undergo electric-field poling.
It is a further object of the present invention to provide a process for producing NLOP film using a solution deposition scheme which results in an electro-optic (EO) film which is not required to have undergone high temperature treatment.
These and other objects are achieved by the present invention which includes a process for producing second-order nonlinear optical film comprising the steps of forming a polycation solution layer comprising a NLO-active cationic polymer on a surface of a substrate, wherein the substrate surface optionally has a pre-existing polyanion layer, forming a polyanion solution layer on the polycation layer, and, alternately forming a plurality of the polycation and the polyanion layers thereon.
Other and further advantages of the present invention are set forth in the description and appended claims.